Which major OS / platforms implement wait morphing?
This question came up when I noticed that there’s no clearcut best practice about whether one should signal a condition variable with mutex locked or not. A typical recommendation is to signal while holding the lock unless profiling shows a substantial performance improvement overhead from unlocking (by removing an extra context switch).
IIUC, the only disadvantage of holding the lock while signalling is the extra two context switches; the advantages are the lower risk of a bug, and easier to achieve real-time predictability.
So, it seems that if wait morphing is implemented, the practice of holding the lock while signalling is strictly better.
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Answer
It’s not supported on Linux. Mark Mossberg investigated it here, and it still holds true as of glibc master today (June 9, 2022).